This invention relates to trip solenoids and more particularly to a solenoid which employs permanent magnets for retaining an armature in a normally retracted position over long periods of time, and which further include an electric coil which is operated to neutralize or cancel the effective magnetic flux of the magnet and permit the armature to move to a second stable position. The electric coil may also be operated, by reversing the flow of current therethrough, to cause the armature to move or be returned to its first stable position.
Such a trip solenoid, as known in the art, commonly employs an armature which moves through or in a working air gap, or a closing air gap with a fixed pole, and commonly provides a shunt path or a secondary flux path including a fixed air gap. The secondary flux path including the fixed air gap has two purposes. First, it determines the point at which the solenoid will be tripped or released by the application of a given current to the electric coil. Second, it provides a flux path for the electric coil which path is parallel to the flux path provided by the pole and frame through the armature, and protects the permanent magnet from de-magnetization.
Commonly trip solenoids employ a single axially oriented holding magnet which is positioned in the space between a portion of the frame and a fixed pole. Commonly the pole itself is provided with a transversely extending portion or an annular portion which is terminated in proximity to the frame defining an air gap therewith, to provide the shunt path. However, such shunt path is commonly not adjustable with respect to reluctance. Additionally, since the holding magnets are commonly formed of cobalt they are relatively costly.